Optical communications are evolving as the chosen technique for data and voice communications. OIC's are often used at the point of transmission, reception and amplification. Optical fibers may be coupled to the OIC to enable the optical connection of the OIC other components of an optical communications system. Typically, planar waveguides are used to efficiently couple light to and from active and passive devices of the OIC. The planar waveguides are often made of relatively high refractive index materials to facilitate desired integration and miniaturization of the OIC. Coupling between the OIC and the optical communication system is often achieved by coupling optical fibers of the system to planar waveguides of the OIC.
While clearly beneficial to the integration and miniaturization of OICs, the planar waveguides commonly used in these circuits do not efficiently couple directly to optical fibers. To this end, planar optical waveguides and optical fiber waveguides used in high-speed and long-haul optical transmission systems often are designed to support a single mode. Stated differently, the waveguides are designed such that the wave equation has one discrete solution; although an infinite number of continuous solutions (propagation constants) may be had. The discrete solution is that of a confined mode, while the continuous solutions are those of radiation modes.
Because each waveguide will have a different discrete (eigenvalue) solution for its confined mode, it is fair to say that two disparate waveguides, such as an optical fiber and a planar waveguide, generally will not have the same solution for a single confined mode. As such, in order to improve the efficiency of the optical coupling, it is necessary to have a waveguide transition region between the planar waveguide of the OIC and the optical fiber. This transition region ideally enables adiabatic compression or expansion of the mode so that efficient coupling of the mode from one type of waveguide to another can be carried out.
As mentioned, optical fibers typically support mode sizes (electromagnetic field spatial distributions) that are much larger, both in the horizontal and vertical directions than modes supported by higher index waveguide structures, such as planar waveguides. Therefore, a challenge is to provide a waveguide transition region that enables adiabatic expansion of the mode so that it is supported by to the optical fiber. Moreover, it is useful to achieve the adiabatic expansion of the mode in both the horizontal and vertical directions.
Fabricating a waveguide to effect adiabatic expansion of the mode in the vertical direction has proven difficult using conventional fabrication techniques. To this end, tapering the thickness of the waveguide to affect the vertical adiabatic expansion of the mode is exceedingly difficult by conventional techniques.
What is needed therefore is a structure for effecting efficient coupling between waveguides having disparate characteristic mode sizes which overcomes the drawbacks of the prior art described above.